Drilling fluid for downhole electrocrushing drilling

ABSTRACT

The disclosure relates to an electrocrushing drilling fluid with an electrocrushing drilling base fluid including a polar oil, a non-polar oil, or a combination thereof and glycerine carbonate. The electrocrushing drilling fluid or base fluid may further include water or glycerin. The electrocrushing drilling fluid may further contain at least one additive. The electrocrushing drilling fluid may have a dielectric constant or dielectric strength of at least a set amount, an electric conductivity less than a set amount, or a combination of these properties. The disclosure further relates to an electrocrushing drilling system containing the electrocrushing drilling fluid and an electrocrushing drill bit.

RELATED APPLICATIONS

This application is a Continuation Application of U.S. patentapplication Ser. No. 15/534,820 filed Jun. 9, 2017, which is a U.S.National Stage Application of International Application No.PCT/US2016/037856 filed Jun. 16, 2016, which designates the UnitedStates.

TECHNICAL FIELD

The present disclosure relates generally to a drilling fluid fordownhole electrocrushing drilling.

BACKGROUND

Electrocrushing drilling uses pulsed power technology to drill aborehole in a rock formation. Pulsed power technology repeatedly appliesa high electric potential across the electrodes of an electrocrushingdrill bit, which ultimately causes the surrounding rock to fracture.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an elevation view of a downhole electrocrushing drillingsystem used in a wellbore environment; and

FIG. 2 illustrates exemplary components of a bottom hole assembly for adownhole electrocrushing drilling system.

DETAILED DESCRIPTION

Electrocrushing drilling may be used to form wellbores in subterraneanrock formations for recovering hydrocarbons, such as oil and gas, fromthese formations. Electrocrushing drilling uses pulsed-power technologyto repeatedly fracture the rock formation by repeatedly deliveringhigh-energy electrical pulses to the rock formation. The fractured rockis removed from the vicinity of the electrocrushing drill bit bydrilling fluid. Although drilling fluids used in conventional mechanicaldrill bit drilling methods also remove fractured rock from the vicinityof the drill bit, thereby allowing drilling to progress downhole,drilling fluids suitable for use with a conventional mechanical drillbit are typically not suitable for use with an electrocrushing drillbit. Electrocrushing drilling fluids have a sufficiently high frequencydependent dielectric constant (also referred to as dielectric constantherein) and dielectric strength, and a sufficiently low electricalconductivity to allow an electrocrushing drill bit to direct an arc ofelectric current through a portion of the rock in the formation, heatingwater and other materials in the rock and causing that rock andsurrounding rock to fracture.

For a given electrocrushing drill bit, the dielectric constant anddielectric strength of the drilling fluid are sufficiently high ifelectric fields generated by the electrocrushing drill bit are directedaway from the drilling fluid and into the rock formation, the rockformation is still fractured, the drilling operation proceeds downholeat at least a set rate, the drilling operation is economically viable,or any combination of these parameters.

For a given electrocrushing drill bit, the electrical conductivity of adrilling fluid is sufficiently low if leakage current from theelectrocrushing drill bit into the drilling fluid is sufficiently lowthat the rock formation is still fractured, the drilling operationproceeds downhole at at least a set rate, the drilling operation iseconomically viable, or any combination of these parameters.

Typically, an electrocrushing drilling fluid will have both sufficientdielectric and electric conductivity properties.

Furthermore, some electrocrushing drilling fluids and electrocrushingdrilling systems using such fluids additionally differ from conventionaldrilling fluids and systems by being more resistant to cavitation causedby electrocrushing drilling or by better mitigating cavitation effects.

An electrocrushing drilling fluid of the present disclosure may includea base fluid containing a polar oil or a non-polar oil, or a combinationthereof and glycerine carbonate. The ratio of polar oil or nonpolar-oil, or combination thereof to glycerine carbonate by volume maybe between 0.85:1 and 1.15:1, between 0.95:1 and 1.05:1, or 1.1.

The electrocrushing drilling fluid or the electrocrushing drilling basefluid may further contain water either before use or after use, as waterfrequently enters the drilling fluid from the formation during use.Salts may also be present because they are dissolved in the water.

The electrocrushing drilling fluid may also contain additives.

Drilling System Using an Electrocrushing Drill Bit and ElectrocrushingDrilling Fluid

Embodiments of the present disclosure and its advantages may beunderstood by referring to FIGS. 1 and 2, where like numbers are used toindicate like and corresponding parts.

FIG. 1 is an elevation view of an exemplary electrocrushing drillingsystem used to form a wellbore in a subterranean formation. AlthoughFIG. 1 shows land-based equipment, downhole tools incorporatingteachings of the present disclosure may be satisfactorily used withequipment located on offshore platforms, drill ships, semi-submersibles,and drilling barges (not expressly shown). Additionally, while wellbore116 is shown as being a generally vertical wellbore, wellbore 116 may beany orientation including generally horizontal, multilateral, ordirectional.

Drilling system 100 includes drilling platform 102 that supports derrick104 having traveling block 106 for raising and lowering drill string108. Drilling system 100 also includes pump 104, which circulateselectrocrushing drilling fluid 122 through feed pipe 124 to kelly 110,which in turn conveys electrocrushing drilling fluid 122 downholethrough interior channels of drill string 108 and through one or moreorifices in electrocrushing drill bit 114. Electrocrushing drillingfluid 122 then circulates back to the surface via annulus 126 formedbetween drill string 108 and the sidewalls of wellbore 116. Typically,during use of drilling system 100, electrocrushing drilling fluid 122carries fractured rock formed by electrocrushing drill bit 114 back tothe surface with it.

Electrocrushing drill bit 114 is attached to the distal end of drillstring 108. In some embodiments, electrocrushing drill bit 114 may besupplied power from the surface. For example, generator 140 may generateelectrical power and provide that power to power-conditioning unit 142.Power-conditioning unit 142 may then transmit electrical energy downholevia surface cable 143 and a sub-surface cable (not expressly shown inFIG. 1) contained within drill string 108. A pulse-generating circuitwithin bottom-hole assembly (BHA) 128 may receive the electrical energyfrom power-conditioning unit 142, and may generate high-energy pulses todrive electrocrushing drill bit 114.

For example, pulsed-power technology may be utilized to repeatedly applya high electric potential, for example up to or exceeding 150 kV, acrossthe electrodes of electrocrushing drill bit 114. Each application ofelectric potential is referred to as a pulse. When the electricpotential across the electrodes of electrocrushing drill bit 114 isincreased enough during a pulse to generate a sufficiently high electricfield, an electrical arc forms through a rock formation at the bottom ofwellbore 116. The arc temporarily forms an electrical coupling betweenthe electrodes, allowing electric current to flow through the arc insidea portion of the rock formation at the bottom of wellbore 116. The arcgreatly increases the temperature and pressure of the portion of therock formation through which the arc flows and the surrounding formationand materials. The temperature and pressure is sufficiently high tobreak the rock itself onto small bits or cuttings.

As electrocrushing drill bit 114 repeatedly fractures rock andelectrocrushing drilling fluid 122 moves the fractured rock uphole,wellbore 116, which penetrates various subterranean rock formations 118,is created. Wellbore 116 may be any hole drilled into a subterraneanformation or series of subterranean formations for the purpose ofexploration or extraction of natural resources such as, for example,hydrocarbons, or for the purpose of injection of fluids such as, forexample, water, wastewater, brine, or water mixed with other fluids.Additionally, wellbore 116 may be any hole drilled into a subterraneanformation or series of subterranean formations for the purpose ofgeothermal power generation.

FIG. 2 illustrates exemplary components of a bottom hole assembly fordownhole electrocrushing drilling system 100. Bottom-hole assembly (BHA)128 may include pulsed-power tool 230. BHA 128 may also includeelectrocrushing drill bit 114. For the purposes of the presentdisclosure, electrocrushing drill bit 114 may be referred to as beingintegrated within BHA 128, or may be referred to as a separate componentthat is coupled to BHA 128.

Pulsed-power tool 230 may be coupled to provide pulsed power toelectrocrushing drill bit 114. Pulsed-power tool 230 receives electricalenergy from a power source via cable 220. For example, pulsed-power tool230 may receive power via cable 220 from a power source on the surfaceas described above with reference to FIG. 1, or from a power sourcelocated downhole such as a generator powered by a mud turbine.Pulsed-power tool 230 may also receive power via a combination of apower source on the surface and a power source located downhole.Pulsed-power tool 230 converts the electrical energy received from thepower source into high-power electrical pulses, and may apply thosehigh-power pulses across electrodes of electrocrushing drill bit 114.For the purposes of the present disclosure, ground ring 250 may also bereferred to generally as an electrode or more specifically as a groundelectrode. In one example, pulsed-power tool 230 may apply thehigh-power pulses across electrode 208 and ground ring 250 ofelectrocrushing drill bit 114. Pulsed-power tool 230 may also applyhigh-power pulses across electrode 210 and ground ring 250 in a similarmanner as described herein for electrode 208 and ground ring 250.

Pulsed-power tool 230 may include a pulse-generating circuit. Such apulse-generating circuit may include high-power capacitors and which mayinclude fuse-protection.

Referring to FIG. 1 and FIG. 2, electrocrushing drilling fluid 122 mayexit drill string 108 via openings 209 surrounding each electrode 208and each electrode 210. The flow of electrocrushing drill fluid 122 outof openings 209 allows electrodes 208 and 210 to be insulated by theelectrocrushing drilling fluid. In some embodiments, electrocrushingdrill bit 114 may include a solid insulator (not expressly shown in FIG.1 or 2) surrounding electrodes 208 and 210 and one or more orifices (notexpressly shown in FIG. 1 or 2) on the face of electrocrushing drill bit114 through which electrocrushing drilling fluid 122 may exit drillstring 108. Such orifices may be simple holes, or they may be nozzles orother shaped features. Because fines are not typically generated duringelectrocrushing drilling, as opposed to mechanical drilling,electrocrushing drilling fluid 122 may not need to exit the drill bit atas high a pressure as the drilling fluid in mechanical drilling. As aresult, nozzles and other features used to increase drilling fluidpressure may not be needed. However, nozzles or other features toincrease electrocrushing drilling fluid 122 pressure or to directelectrocrushing drilling fluid may be included for some uses.

Drilling fluid 122 is typically circulated through drilling system 100at a flow rate sufficient to remove fractured rock from the vicinity ofelectrocrushing drill bit 114 in sufficient quantities within asufficient time to allow the drilling operation to proceed downhole atleast at a set rate. In addition, electrocrushing drilling fluid 122 maybe under sufficient pressure at a location in wellbore 116, particularlya location near a hydrocarbon, gas, water, or other deposit, to preventa blowout.

Electrodes 208 and 210 may be at least 0.4 inches apart from ground ring250 at their closest spacing, at least 1 inch apart at their closestspacing, at least 1.5 inches apart at their closest spacing, or at least2 inches apart at their closest spacing. If drilling system 100experiences vaporization bubbles in electrocrushing drilling fluid 122near electrocrushing drill bit 114, the vaporization bubbles may havedeleterious effects. For instance, vaporization bubbles near electrodes208 or 210 may impede formation of the arc in the rock. Electrocrushingdrilling fluids 122 may be circulated at a flow rate also sufficient toremove vaporization bubbles from the vicinity of electrocrushing drillbit 114.

In addition, electrocrushing drill bit 114 may include ground ring 250,shown in part in FIG. 2. Although not all electrocrushing drill bits 114may have ground ring 250, if it is present, it may contain passages 260to permit the flow of electrocrushing drilling fluid 122 along with anyfractured rock or bubbles away from electrodes 208 and 210 and uphole.

Referring again to FIG. 1, at the surface, recirculated or spentelectrocrushing drilling fluid 122 exits annulus 126. Recirculated orspent electrocrushing drilling fluid 122 may simply be directed backinto drill string 108 or may otherwise be processed using only some ofthe equipment shown in FIG. 1. However, in the drilling system 100,electrocrushing drilling fluid 122 is conveyed to one or more fluidprocessing units 150 via an interconnecting flow line 130. After passingthrough fluid processing units 150, cleaned electrocrushing drillingfluid 122 is deposited into retention pit 132. Although fluid processingunit 150 is illustrated in FIG. 1 near the outlet of the wellbore 116,fluid processing unit 150 may be located at a distance from the outletof wellbore 116.

Drilling system 100 may further include mixing hopper 134 communicablycoupled to or otherwise in fluidic communication with retention pit 132.Mixing hopper 134 may include, but is not limited to, mixers and relatedmixing equipment. Mixing hopper 134 may be used to add additives toelectrocrushing drilling fluid 122.

Drilling fluid 122 is typically circulated through drilling system 100at a flow rate sufficient to remove fractured rock from the vicinity ofelectrocrushing drill bit 114 in sufficient quantities within asufficient time to allow the drilling operation to proceed downhole atat least a set rate.

In addition, electrocrushing drilling fluid 122 may be under sufficientpressure at a location in wellbore 116, particularly a location near ahydrocarbon, gas, water, or other deposit, to prevent a blowout.

If drilling system 100 experiences cavitation in electrocrushingdrilling fluid 122 near electrocrushing drill bit 112, the may havevarious deleterious effects. For instance, cavitation bubbles nearelectrodes 208 and 210 may impede formation of the arc in the rock.Electrocrushing drilling fluids 122 may be circulated at a flow ratealso sufficient to remove cavitation bubbles from the vicinity ofelectrocrushing drill bit 114 in sufficient quantities within asufficient time to allow the drilling operation to proceed downhole atat least a set rate and to avoid damage to drill bit 112 from thecavitation bubbles, for instance damage resulting from the cavitationbubbles reentering drill bit 112.

Electrocrushing Drilling Fluid

To limit discharge of the electric field through electrocrushingdrilling fluid 122 and allow more electrical current to flow into therock at the end of wellbore 116, an electrically insulatingelectrocrushing drilling fluid 122 with a high dielectric constant and ahigh dielectric strength at a particular operating frequency may beused. An electrically insulating electrocrushing drilling fluid 122restricts the movement of electrical charges, and therefore, the flow ofelectrical current through the electrocrushing drilling fluid 122. Ahigh dielectric constant and high dielectric strength decreaseelectrical discharge through electrocrushing drilling fluid 122. Thedielectric constant of the downhole fluid indicates the ability of theelectrocrushing drilling fluid to store electrical energy when exposedto an electric field, such as the potential created by electrocrushingdrill bit 114, while the dielectric strength of the downhole fluidindicates a voltage level to which electrocrushing drilling fluid 122may be exposed before experiencing electrical breakdown, or a loss ofits electrically insulating properties.

Electrocrushing drilling fluid 122 may be formulated to have:

-   -   i) at least a set dielectric constant, such as at least 6, at        least 10, at least 12, or at least 13 (at 100 kHz frequency),    -   ii) at least a set dielectric strength, such as at least 100        kV/cm, at least 150 kV/cm, or at least 330 kV/cm (at 10        microseconds rise time),    -   iii) less than a set electric conductivity, such as less than        10⁻⁴ mho/cm, or less than 10⁻⁵ mho/cm,    -   or any combinations thereof.

Electrocrushing drilling fluid 122 includes an electrocrushing drillingbase fluid and may include one or more additives. Generally, theelectrocrushing drilling base fluid may be present in an amountsufficient to form a pumpable electrocrushing drilling fluid. By way ofexample, the electrocrushing drilling base fluid may be present inelectrocrushing drilling fluid 122 in an amount in the range of from 20%to 99.99% by volume of electrocrushing drilling fluid 122.

The electrocrushing drilling base fluid includes a polar oil, anon-polar oil, or a combination thereof and glycerine carbonate.

Polar oil, if present, may include a combination of polar oils. Polaroils may include a vegetable oil, such as castor oil, a ester oil, suchas a polyol ester or monoester oil, or any combination thereof. Polyolester and monoester oils may provide greater hydrolytic stability, lowerviscosity, or both as compared to vegetable oils.

A polar oil may include any ester of a carboxylic acid, such as acarboxylic acid with between 8 and 20 carbons, and 0, 1, or 2 moles ofunsaturation. The carboxylic acid may be esterified using an alcohol,such as methanol, isopropanol, or 2-ethylhexanol.

A polar vegetable oil may include esters of vegetable oils such asesters of palm oil, palm kernal oil, rape seed oil, soybean oil, stericacid, oleic acid, and linoleic acid, and any combinations thereof.

A polyol ester oil may include a glycol ester oil, such as a neopentylglycol diester.

A monoester oil may include hexanyl propanoate and isomers, hexanylbutyrate and isomers, hexanyl hexanoate and isomers, hexanyl octanoateand isomers, hexanyl decanoate and isomers, hexanyl laureate andisomers, hexanyl palmitate and isomers, hexanyl hexadecanoate andisomers, hexanyl stearate and isomers, octanyl propanoate and isomers,octanyl butyrate and isomers, octanyl hexanoate and isomers, octanyloctanoate and isomers, octanyl decanoate and isomers, octanyl laureateand isomers, octanyl palmitate and isomers, octanyl hexadecanoate andisomers, octanyl stearate and isomers, decanyl propanoate and isomers,decanyl butyrate and isomers, decanyl hexanoate and isomers, decanyloctanoate and isomers, decanyl decanoate and isomers, decanyl laureateand isomers, decanyl palmitate and isomers, decanyl hexadecanoate andisomers, decanyl stearate and isomers, dodecanyl propanoate and isomers,dodecanyl butyrate and isomers, dodecanyl hexanoate and isomers,dodecanyl octanoate and isomers, dodecanyl decanoate and isomers,dodecanyl laureate and isomers, dodecanyl palmitate and isomers,dodecanyl hexadecanoate and isomers, dodecanyl stearate and isomers,tetradecanyl propanoate and isomers, tetradecanyl butyrate and isomers,tetradecanyl hexanoate and isomers, tetradecanyl octanoate and isomers,tetradecanyl decanoate and isomers, tetradecanyl laureate and isomers,tetradecanyl palmitate and isomers, tetradecanyl hexadecanoate andisomers, tetradecanyl stearate and isomers, hexadecanyl propanoate andisomers, hexadecanyl butyrate and isomers, hexadecanyl hexanoate andisomers, hexadecanyl octanoate and isomers, hexadecanyl decanoate andisomers, hexadecanyl laureate and isomers, hexadecanyl palmitate andisomers, hexadecanyl hexadecanoate and isomers, hexadecanyl stearate andisomers, octadecanyl propanoate and isomers, octadecanyl butyrate andisomers, octadecanyl hexanoate and isomers, octadecanyl octanoate andisomers, octadecanyl decanoate and isomers, octadecanyl laureate andisomers, octadecanyl palmitate and isomers, octadecanyl hexadecanoateand isomers, octadecanyl stearate and isomers, icosanyl propanoate andisomers, icosanyl butyrate and isomers, icosanyl hexanoate and isomers,icosanyl octanoate and isomers, icosanyl decanoate and isomers, icosanyllaureate and isomers, icosanyl palmitate and isomers, icosanylhexadecanoate and isomers, icosanyl stearate and isomers, docosanylpropanoate and isomers, docosanyl butyrate and isomers, docosanylhexanoate and isomers, docosanyl octanoate and isomers, docosanyldecanoate and isomers, docosanyl laureate and isomers, docosanylpalmitate and isomers, docosanyl hexadecanoate and isomers, docosanylstearate, and any combinations thereof.

Specific suitable non-polar oils include PETROFREE® (Halliburton, Tex.,US), which is an ester of 2-ethylhexanol reacted with palm kernel fattyacid, and PETROFREE LV® (Halliburton, Tex., US), which is an ester of2-ethylhexanol reacted with C6 to C10 fatty acids.

Non-polar oils typically have a high dielectric strength and a lowelectric conductivity, making them a suitable replacement for alkylenecarbonate in electrocrushing drilling base fluids. However, non-polaroils have a low dielectric constant, may be included with othercomponents with a higher dielectric constant in an electrocrushingdrilling base fluid. A non-polar oil suitable for use in anelectrocrushing drilling base fluid of the present disclosure includescombinations of non-polar oils. Suitable non-polar oils include mineraloils, diesel oils or fuels, paraffin-based oils, oils containingbranched and linear aliphatic hydrocarbons having between 8 and 26carbon atoms and a boiling point in the range of 120° C. to 380° C.,oils containing hydrocarbons having between 10 and 16 carbon atoms and aviscosity of 1.5 to 2 cSt at 40° C. Any of the non-polar oils orcombinations thereof may have a viscosity of less than 4 cSt at 40° C.

Combinations of polar oils and non-polar oils may also be used.

Polar oils tend to have dielectric constants or dielectric strengthsthat are too low for electrocrushing drilling. As a result, glycerinecarbonate may be added to the polar oil to improve these propertiesbecause it has a high dielectric constant and moderate dielectricstrength and is more oil soluble than many other alkylene carbonates.However, the amount of glycerine carbonate in the electrocrushingdrilling base oil may be limited by its high electric conductivity.Other alkylene carbonates may also be present in the electrocrushingdrilling base oil, or it may have no or less than 0.1% by weight orvolume other alkylene carbonates.

The electrocrushing drilling fluid or base fluid also contains water.Water has a low viscosity and a high dielectric strength, but it alsohas a high electric conductivity, thus potentially limiting itsproportional volume in an electrocrushing drilling fluid or base fluid.The electric conductivity of water further increases if salts aredissolved in the water, a frequent occurrence during drilling.

Water also has a highly temperature-variable dielectric constant thatdecreases with temperature and thus which may also limit water'sproportional volume in an electrocrushing drilling fluid or base fluidbecause the electrocrushing drilling fluid typically experiences hightemperatures in the vicinity of the electrocrushing drill bit.

The electrocrushing drilling fluid or base fluid may further includeglycerin. Glycerin has a high dielectric constant and low electricconductivity, but also low dielectric strength, thus potentiallylimiting its proportional volume in an electrocrushing drilling fluid orbase fluid.

One or more electrical additives may change one or more electricalproperties of the electrocrushing drilling base fluid. For instance, anelectrical additive may change a dielectric property of theelectrocrushing drilling base fluid. Such additives may include mica inany of its various forms such as muscovite, phlogopite, leidolite,fluorophlogopite, glass-bonded mica, and biotite,polytetrafluoroethylene, other chemical variants of tetrafluoroethylene,glass or a composition of glass including fused silica andalkali-silicate, polystyrene, polyethylene, diamond, lead zirconatetitanate (PZT), sodium chloride crystalline, potassium bromidecrystalline, silicone oil, benzene, and any combinations thereof. Theelectrical additive may be present in an electrocrushing drilling fluidin an amount sufficient for a particular drilling system, formation, orcombination thereof. The type of electrical additive or combination ofelectrical additives in an electrocrushing drilling fluid may also bebased at least partially upon a particular drilling system, formation,or combination thereof.

The electrocrushing drilling fluid may further include additives used inconventional drilling fluids. These additives may provide properties tothe electrocrushing drilling fluid similar to the properties theyprovide to conventional drilling fluids. However, some additives used inconventional drilling fluids may not be suitable for an electrocrushingdrilling fluid due to their effects on dielectric constant, dielectricstrength, or electric conductivity, or because they are not compatiblewith an electrocrushing drill bit.

Additives may include a lost circulation prevention material, such as abridging material or a fluid loss control agent, a rheology modifier,such as a viscosifier or a thinner, a weighting agent, a solids wettingagent, an acid or H₂S scavenger, a lubricant other additives, and anycombinations thereof.

Lost circulation materials are capable of reducing the amount of wholedrilling fluid that is lost to fractures and natural caverns during thedrilling process. Lost circulation materials include mica, fibers, andhard organic materials, such as nutshells. The lost circulation materialmay be present in an electrocrushing drilling fluid in an amountsufficient for a particular drilling system, formation, or combinationthereof. The type of lost circulation material or combination of lostcirculation materials in an electrocrushing drilling fluid may also bebased at least partially upon a particular drilling system, formation,or combination thereof.

Lost circulation materials include bridging materials, which bridgeacross pores and fractures in the formation and help prevent loss ofdrilling fluid into the formation. Bridging materials may includecalcium carbonate, salt supensions, resins, BARACARB® (Halliburton,Tex., US) size-ground marble, N-SEAL™ (Halliburton, Tex., US) extrusionspun mineral fiber or similar materials.

Fluid loss control agents, which help control loss of the liquid portionof the drilling fluid into the formation, may also be used in theelectrocrushing drilling fluid . Fluid loss control agents include claysand polymers, such as synthetic polymers or natural polymers, such aslignitic polymers.

Rheology modifiers change the flow properties of the electrocrushingdrilling fluid. Rheology modifiers may, for instance, change the shearproperties or viscosity of the drilling fluid. The rheology modifier maybe present in the electrocrushing drilling fluid in an amount sufficientfor a particular drilling system, formation, or combination thereof. Thetype of rheology modifier or combination of rheology modifiers in theelectrocrushing drilling fluid may also be based at least partially upona particular drilling system, formation, or combination thereof.

Thinners are a type of rheology modifier that decrease the viscosity ofa drilling fluid. In drilling fluids that experience flocculation, suchas drilling fluids containing some clays, thinners may also bedeflocculants. Electrocrushing drilling may benefit from a low viscositydrilling fluid, such that thinners may be a particularly usefuladditive.

Viscosifiers increase the viscosity of a drilling fluid. A viscosifiermay be used in the drilling fluid to impart a sufficient carryingcapacity or thixoropy or both to the drilling fluid, enabling thedrilling fluid to transport and prevent settling of fractured rock orweighting materials, or both. Suitable viscosifiers include organophilicclays such as GELTONE® II viscosifier (Halliburton, Tex., US), polymericviscosifiers, such as BARARESIN® VIS viscosifier (Halliburton, Tex.,US), long chain fatty acids, dimer/trimer/tetramer fatty acids (RM-63™viscosifier, Halliburton, Tex., US), and any combinations thereof.

The electrocrushing drilling fluid may have a viscosity at surfacetemperature and pressure sufficient to allow it to suspend any particlesadditives, such as barite or a dielectric modifier, while still allowingit to be pumped downhole. In the wellbore, the drilling fluid maymaintain a viscosity sufficient to allow it to suspend any particleadditives, while still allowing it to circulate through and out of thewellbore. The electrocrushing drilling fluid may further maintain aviscosity upon return to surface pressure or temperature sufficient toallow it to exit the wellbore. The electrocrushing drilling fluid mayalso further maintain its viscosity to allow it to continue to suspendany particles additives, such as barite, until it reaches a holdingtank, through any cleaning or testing process, or until it is returnedto a wellbore, as applicable.

Emulsifiers help create a mixture of two immiscible liquids, such as anoil-based liquid and an aqueous liquid. Suitable emulsifiers includepolyaminated fatty acids. Electrocrushing drilling fluid 122 is aninvert emulsion and thus may particularly benefit from an emulsifier.The emulsifier may be present in electrocrushing drilling fluid 122 inan amount sufficient for a particular drilling system, formation, orcombination thereof. The type of emulsifier or combination of emulsifierin electrocrushing drilling fluid 122 may also be based at leastpartially upon the immiscible components of electrocrushing drillingfluid 122, a particular drilling system, formation, or combinationthereof.

Weighting agents increase the density of an electrocrushing drillingfluid without being dissolved in it. Suitable weighting agents includebarite, hematite, ilmenite, manganese tetraoxide, and any combinationsthereof. The weighting agent may be present in an electrocrushingdrilling fluid in an amount sufficient for a particular drilling system,formation, or combination thereof. The type of weighting agent orcombination of weighting agents in an electrocrushing drilling fluid mayalso be based at least partially upon a particular drilling system,formation, or combination thereof. Typically, for electrocrushingdrilling, the amount of weighing agent present is sufficient to maintainan electrocrushing drilling fluid density between 8 lb/gallon and 21lb/gallon.

Other additives may include corrosion inhibitors, defoamers, shalestabilizers, lubricants, wetting agents, dispersing agents, shaleinhibitors, pH-control agents, filtration-control agents, alkalinitysources such as lime and calcium hydroxide, salts, foamers,viscosifiers, thinners, deflocculents, or any combinations thereof. Theother additives may be present an electrocrushing drilling fluid in anamount sufficient for a particular drilling system, formation, orcombination thereof. The type of other additives or combination of otheradditives in an electrocrushing drilling fluid may also be based atleast partially upon a particular drilling system, formation, orcombination thereof.

Some additives, such as lignitic fluid loss control agents andpolaminated fatty acid emulsifiers, may have synergistic effects.

In addition to conventional uses for additives, some additives may havea further effect or may be added solely for the effect of rendering theelectrocrushing drilling fluid or the drilling system more resistant tocavitation caused by electrocrushing drilling or by better mitigatingcavitation effect as compared to conventional drilling fluids orelectrocrushing drilling fluids without the additive.

EXAMPLES

The following examples are provided to further illustrate certainaspects of the invention and should not be interpreted as disclosing theinvention as a whole. In particular, substitutions within these examplesin accordance with the above disclosure are also encompassed by theinvention.

In the following examples, lb/bbl is one pound per 42 gallons of totalelectrocrushing drilling fluid.

Example 1: Electrocrushing Drilling Fluid With Polar Oil and GlycerineCarbonate

The following components were mixed at to yield a suitableelectrocrushing drilling fluid:

2 lb/bbl Adapta® (Halliburton, Tex., US) methylstyrene/acrylatecopolymer lost circulation material

8 lb/bbl LE Supermul (Halliburton, Tex., US) polyaminated fatty acidemulsifier

2 lb/bbl Rhemod L® (Halliburton, Tex., US) modified fatty acidcontaining dimeric and trimeric fatty acids viscosifier

92.5 lb/bbl glycerine carbonate

56 lb/bbl Petrofree® (Halliburton, Tex., US) ester polar oil

119.8 lb/bbl Baroid® (Halliburton, Tex., US) barite weighting agent

25.28 lb/bbl water

10.14 lb/bbl calcium chloride.

Example 2: Electrocrushing Drilling Fluid With Non-Polar Oil andGlycerine Carbonate

The following components were mixed at to yield a suitableelectrocrushing drilling fluid:

2 lb/bbl Adapta® (Halliburton, Tex., US) methylstyrene/acrylatecopolymer lost circulation material

8 lb/bbl LE Supermul (Halliburton, Tex., US) polyaminated fatty acidemulsifier

2 lb/bbl Rhemod L® (Halliburton, Tex., US) modified fatty acid containgdimeric and trimeric fatty acids viscosifier

92.5 lb/bbl glycerine carbonate

55.73 lb/bbl diesel (Halliburton, Tex., US) non-polar oil

119.8 lb/bbl Baroid® (Halliburton, Tex., US) barite weighting agent

25.28 lb/bbl water

10.14 lb/bbl calcium chloride.

Example 3—Methods of Measuring Dielectric Constant and ElectricConductivity of an Electrocrushing Drilling Fluid

The dielectric constant and electric conductivity of an electrocrushingdrilling fluid, such as one disclosed herein, may be measured using anetwork analyzer or inductance, capacitance and resistance (LCR) meterequipped with a particular fixture, such as a liquid test fixture, toallow the measurement. Suitable measurement equipment includes thatproduced by Agilent (Santa Clara, Calif.) and Keysight (Santa Rosa,Calif.), particularly the Keysight 16452A Liquid Text Fixture and theAgilent E4991B Impedance Analyzer. Such equipment may be used inaccordance with manufacturer instructions.

One of ordinary skill in the art may determine how to determine thefrequency dependent dielectric constant and the electric conductivity ofan electrocrushing drilling fluid using the above-identified or similarequipment by applying the following principles, typically as implementedin a programmed computer receiving data from equipment.

The frequency dependent complex dielectric constant of a fluid may bedetermined by the following equation, which includes its real andimaginary parts:

${ɛ(\omega)} = {{{ɛ_{r}^{\prime}(\omega)} + {j\;{ɛ_{r}^{''}(\omega)}}} = {{{ɛ_{r}(\omega)}ɛ_{0}} + {j\;\frac{\sigma(\omega)}{\omega}}}}$where:

-   -   ϵ(ω) is the frequency dependent complex dielectric constant    -   ϵ_(r)′(ω) is the real part of the dielectric constant    -   ϵ_(r)″(ω) is the imaginary part of the dielectric constant    -   and j=√{square root over (−1)}.

In addition,ϵ_(r)′(ω)=ϵ_(r)(ω)ϵ₀

${ɛ_{r}(\omega)} = \frac{ɛ_{r}^{\prime}(\omega)}{ɛ_{0}}$so thatwhere:

-   -   ϵ_(r)(ω) is the frequency dependent dielectric constant of the        fluid    -   and ϵ₀ is the permittivity of free space=8.85418782×10⁻¹²        Farads/Meter.

Additionally,

${ɛ_{r}^{''}(\omega)} = \frac{\sigma(\omega)}{\omega}$and therefore,σ(ω)=ωϵ_(r)″(ω)Where σ(ω) is the frequency dependent electric conductivity of thefluid.

Example 4—Methods of Measuring Dielectric Strength of an ElectrocrushingDrilling Fluid

Equipment suitable to measure the dielectric strength of anelectrocrushing drilling fluid over a given rise time is generally notcommercially available, but it may be constructed by attaching anadjustable voltage supply to electrodes immersed in the fluid andotherwise electrically insulated from one another such that an electricfield ({right arrow over (E)}) may be created between the electrodesusing the adjustable voltage supply when it applies a voltage (V) acrossthe electrodes. The electrodes may be any shape, such as spheres orplates. The electrodes are separated by a distance D. An ammeter islocated in the circuit between the adjustable voltage supply and thenegative electrode. The adjustable voltage supply may be used to providehigh voltage pulses with an adjustable peak voltage and rise time.

{right arrow over (E)} is determined using the following equation:

$\overset{\rightarrow}{E} = \frac{V}{D}$

{right arrow over (E)} electric field is a vector quantity and thedirection of the electric field goes from the positive electrode towardthe negative one. One may alter the electric field by adjusting either Vor D.

For a given rise time, for a series of voltage pulses across theelectrodes and through the electrocrushing drilling fluid under test,the peak electric field ({right arrow over (E)}) for each pulse isincreased until conduction current is observed in the ammeter.Conduction current indicates electrical breakdown of the liquid. For agiven rise time, the electric field the electrocrushing drilling fluidcan support without breakdown (and hence measurement of a current byammeter) is its dielectric strength for that rise time. By varying therise time and electric field and noting the electric field/rise timecombinations where breakdown occurs, one may determine the dielectricstrength of the electrocrushing drilling fluid as a function of risetime.

The disclosure provides at least two embodiments A and B.

-   -   A. An electrocrushing drilling fluid including a electrocrushing        drilling base fluid including a polar oil, a non-polar oil or a        combination thereof, and glycerine carbonate, wherein the        electrocrushing drilling base fluid has a dielectric constant of        at least 6 at 100 kHz frequency.    -   B. An electrocrushing drilling system including a drill string,        an electrocrushing drill bit attached to the downhole end of the        drill string for fracturing rock in a formation, and an        electrocrushing drilling fluid that circulates downhole through        the drill string and then moves the fractured rock uphole. The        electrocrushing drilling fluid is that of embodiment A.

Both of embodiments A and B may have one or more of the followingadditional elements in any combination, unless clearly mutuallyexclusive: i) the electrocrushing drilling base fluid may have adielectric strength of at least 100 kV/cm, at least 150 kV/cm, or atleast 330 kV/cm at 10 microseconds rise time; ii) the electrocrushingdrilling base fluid may have an electric conductivity of less than 10⁻⁴or less than 10⁻⁵ mho/cm; iii) the electrocrushing drilling base fluidmay have an oil:glycerine carbonate ratio of between 0.85: 1 and 1.15:1(v:v); iv) the polar oil may include a vegetable oil, an ester oil, orany combinations thereof; v) the non-polar oil may include a mineraloil, a diesel oil or fuel, a paraffin-based oil, an oil containingbranched and linear aliphatic hydrocarbons having between 8 and 26carbon atoms and a boiling point in the range of 120° C. to 380° C., anoil containing hydrocarbons having between 10 and 16 carbon atoms and aviscosity of 1.5 to 2 cSt at 40° C., or any combinations thereof; vi)the electrocrushing drilling fluid may further include water; v) theelectrocrushing drilling fluid may further include glycerin; vi) theelectrocrushing drilling fluid may further include at least one additiveincluding an electrical additive, a lost circulation material, arheology modifier, a weighting agent, an emulsifier, a corrosioninhibitor, a defoamer, a shale stabilizer, a lubricant, a wetting agent,a dispersing agent, a shale inhibitor, a pH-control agent, afiltration-control agent, an alkalinity source, a foamer, a viscosifier,a thinner, a deflocculent, or any combinations thereof; vii) theelectrocrushing drilling base fluid may have a dielectric strength of atleast 330 kV/cm at 10 microseconds rise time.

Although the present disclosure has been described with severalembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosureencompasses such various changes and modifications as falling within thescope of the appended claims.

The invention claimed is:
 1. An electrocrushing drilling systemcomprising: a drill string; an electrocrushing drill bit attached to thedownhole end of the drill string for fracturing rock in a formation; andan electrocrushing drilling fluid that circulates downhole through thedrill string and then moves the fractured rock uphole, wherein theelectrocrushing drilling fluid comprises an electrocrushing drillingbase fluid comprising: a polar oil or a non-polar oil or a combinationthereof; and glycerine carbonate; wherein the electrocrushing drillingbase fluid has a dielectric constant of at least 6 at 100 kHz frequency;and wherein the electrocrushing drilling base fluid has an electricconductivity of less than 10⁻⁵ mho/cm.
 2. The electrocrushing drillingsystem of claim 1, wherein the electrocrushing drilling base fluid has adielectric strength of at least 100 kV/cm at 10 microseconds rise time.3. The electrocrushing drilling system of claim 1, wherein theelectrocrushing drilling base fluid has an electric conductivity of lessthan 10⁻⁴ mho/cm.
 4. The electrocrushing drilling system of claim 1,wherein the electrocrushing drilling base fluid has an oil:glycerinecarbonate ratio of between 0.85:1 and 1.15:1 (v:v).
 5. Theelectrocrushing drilling system of claim 1, wherein the polar oilcomprises a vegetable oil, an ester oil, or any combinations thereof. 6.The electrocrushing drilling system of claim 1, wherein the non-polaroil comprises a mineral oil, a diesel oil or fuel, a paraffin-based oil,an oil containing branched and linear aliphatic hydrocarbons havingbetween 8 and 26 carbon atoms and a boiling point in the range of 120 °C. to 380 ° C., an oil containing hydrocarbons having between 10 and 16carbon atoms and a viscosity of 1.5 to 2 cSt at 40° C., or anycombinations thereof.
 7. The electrocrushing drilling system of claim 1,further comprising water.
 8. The electrocrushing drilling system ofclaim 1, further comprising glycerin.
 9. The electrocrushing drillingsystem of claim 1, further comprising at least one additive comprisingan electrical additive, a lost circulation material, a rheologymodifier, a weighting agent, an emulsifier, a corrosion inhibitor, adefoamer, a shale stabilizer, a lubricant, a wetting agent, a dispersingagent, a shale inhibitor, a pH-control agent, a filtration-controlagent, an alkalinity source, a foamer, a viscosifier, a thinner, adeflocculent, or any combinations thereof.